Atmospheric pressure glow discharge treatment of polymeric supports to promote adhesion for photographic applications

ABSTRACT

The present invention is a method for treating a polyester support such as polyethylene naphthalate or polyethylene terephthalate. The treatment is carried out at near atmospheric pressure in a gas of helium and optionally nitrogen and/or oxygen. The treatment uses anodized aluminum electrodes and an atmospheric glow discharge results when the electrodes are connected to an RF generator and spaced about 2 mm apart. The process and apparatus improve adhesion of subsequently coated emulsions on the polyester support at high speeds and relatively low power by selecting a frequency of 40 kHz to 500 kHz.

FIELD OF THE INVENTION

This invention describes an improved process for treating photographicsupport with electrical discharges at atmospheric pressure to promotethe adhesion of subsequent coated layers.

BACKGROUND OF THE INVENTION

Corona discharges are used widely in industry to promote adhesionbetween various materials. In manufacturing photographic products thereis a large body of literature describing various applications of coronasto make aqueous and non-aqueous coatings adhere to substrate materials.Almost all of these coronas are produced by applying a high voltage(approximately 5-10 kV), relatively high frequency (10 kHz) signal toelectrodes in air at atmospheric pressure. See, for example, U.S. Pat.No. 4,241,169; U.S. Pat. No. 4,701,403; U.S. Pat. No. 4,087,574; U.S.Pat. No. 4,429,032; U.S. Pat. No. 4,363,872; U.S. Pat. No. 4,229,523;U.S. Pat. No. 4,394,442; U.S. Pat. No. 3,411,908; U.S. Pat. No.3,531,314; U.S. Pat. No. 3,582,339; U.S. Pat. No. 3,607,345; U.S. Pat.No. 3,630,742; U.S. Pat. No. 3,860,427; U.S. Pat. No. 3,874,877; U.S.Pat. No. 3,888,753; U.S. Pat. No. 4,055,685; U.S. Pat. No. 4,518,681;U.S. Pat. No. 5,004,669; FR 76 13034; EP Application No. 92303556.2.There are limitations to the usefulness of corona treatments, however.Coronas produce locally energetic discharges, known commonly asstreamers, and these streamers may cause a non-uniform level oftreatment. They may also be related to an inhomogeneous loss of redspeed in photographic emulsions which produces a mottle defect.Furthermore, coronas appear to be effective at promoting adhesion ofcoatings to polyethylene, but are relatively ineffective at promotingthe adhesion of layers to various polyester supports such aspolyethylene terephthalate (PET), polyethylene naphthalate (PEN), etc.

A more controllable and effective way of preparing polymers for coatingis with low pressure glow discharge treatments. Glow discharges are, bynature, very diffuse and homogeneous, producing a more uniformtreatment. Moreover, by controlling the gas it is possible to improvethe adhesion of photographic layers to materials such as polyesters aswell as polyethylene. See, for example, U.S. Pat. No. 4,993,267; U.S.Pat. No. 3,837,886; U.S. Pat. No. 4,451,497. A major disadvantage inglow discharge treatments done at reduced pressures is the problem ofmaintaining a low pressure at the treatment station. It is necessary touse either a batch process, in which the support is loaded into achamber and the air is removed, or an in-line process, which requiresthat the support pass through a differential pressure region. In thefirst case, the support must go through an additional off-line stepbefore the coatings are applied. This is unattractive from aproduct-flow perspective and requires additional capital. The secondchoice is difficult and expensive to implement because of the very tighttolerances needed to maintain the pressure differentials in thetransport path. This requires expensive and complicated hardware andpumps. The closer to atmospheric pressure that these treatments can bedone, the simpler and less costly the process becomes.

It is known that under the right conditions, stable diffuse glowdischarges can be produced at atmospheric pressures. Articles thatdiscuss stable atmospheric glow discharges are: S. Kanazawa, M. Kogoma,T. Moriwaki, and S. Okazaki, J. Phys. D: Appl. Phys 21 (1988) 838-840;S. Kanazawa, M. Kogoma, S. Okazaki, and T. Moriwaki, Nuclear Instrumentsand Methods in Physics Research B37/38 (1989) 842-845; T. Yokoyama, M.Kogoma, S. Kanazawa, T. Moriwaki, and S. Okazaki, J. Phys. D: Appl.Phys. 23 (1990) 374-377; T. Yokoyama, M. Kogoma, T. Moriwaki, and S.Okazaki, J. Phys. D: Appl. Phys. 23 (1990) 1125-1128; and A. Nagata, S.Takehiro, H. Sumi, M. Kogoma, S. Okazaki, and Y. Horiike, Proc. Jpn.Symp. Plasma Chem 2 (1989) 109-112. This area has been limited anddirected primarily at etching of photoresist and deposition ofmaterials. However, there are references to treatments for adhesion (WO94/28568). Many reports indicate that a reliable method of producingdiffuse glow discharges at atmospheric pressures is to use helium as thedischarge gas. The work reported in the literature has been reproducedand found to be reliable. It has also been found that very small amountsof reactive gases, such as a few percent nitrogen or oxygen, willextinguish an atmospheric helium discharge. However, we have found thatby using trace amounts of active gases in a novel discharge device, atcertain frequencies stable atmospheric pressure discharges can beproduced which can dramatically improve the adhesion of photographicemulsions to difficult to coat materials such as polyethylene, PET, andPEN.

In U.S. Ser. No. 08/299,776 filed Sep. 1, 1994, we describe a method oftreating a polymeric support comprising a first electrode having a firstsurface, the first electrode having a plurality of spaced apart holesadjoining the first surface, positioning a second electrode having asecond surface spaced apart from the first surface of the firstelectrode, pumping gas through the holes wherein the gas is greater thanor equal to atmospheric pressure, the gas comprising helium andoptionally oxygen and/or nitrogen, coupling a power supply to the firstelectrode having a frequency of 10 kHz to 50 mHz, and positioning a webbetween the first surface of the first electrode and the second surfaceof the second electrode wherein the polymeric web is subjected toatmospheric glow discharge to improve the adhesive properties.

The above method has been found to be very useful, but it is quiteimportant, in photographic systems, to be able to run film through atextremely fast rates such as 5 ft. per minute or higher and atcomparatively low power densities, such as 5 watts per square centimeteror less. For treatment purposes, the power density is defined as thetotal power delivered to the treatment electrode divided by the area ofthe treatment zone.

The present invention allows one to treat polymeric surfaces with astable atmospheric glow discharge so that adhesion of photographicemulsions is improved while operating at high speeds and relatively lowpower requirements.

SUMMARY OF THE INVENTION

The present invention is a method of treating a polymeric support. Themethod includes providing a first electrode having a first surface, thefirst electrode having a plurality of spaced apart holes adjoining thefirst surface, the first surface being insulated. A second electrodehaving a second surface is positioned in a spaced apart relationshipfrom the first surface of the first electrode. Gas is pumped through theplurality of holes at a pressure greater than or equal to atmosphericpressure. The gas comprises helium and optionally oxygen and/or,nitrogen. A power supply is coupled to the first electrode, the powersupply has a frequency of between 40 kHz to about 500 kHz. A web ispositioned between the first surface of the first electrode and thesecond surface of the second electrode wherein the polymeric web issubjected to atmospheric glow discharge to improve the adhesiveproperties. The ratio of the speed of the web in feet per minute to thepower density provided at the treatment station in W/cm² is 1:1 orhigher.

The present invention provides the advantage of improving the adhesiveproperties of a polyester substrate using a glow discharge device thatoperates at atmospheric pressures while maintaining a high speed ofsupport treatment at relatively low power density.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a prior art device used to obtain anatmospheric glow discharge.

FIG. 2 shows an electrode configuration of the present invention for thecontinuous treatment of a moving web.

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following detailed description and appended claims inconnection with the preceding drawings and description of some aspectsof the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a prior art set-up used to obtain a near atmosphericpressure glow discharge. Two solid square aluminum electrodes 10 and 11,one of which was anodized (electrode 10), were used to treat fullyoriented PET and oriented annealed PEN in helium and in mixtures ofhelium and oxygen and/or nitrogen. The electrodes 10 and 11 were 7.5 by7.5 cm and were 2 mm apart. They were powered by an RF generator 12operating at 13.56 mHz. With a mixture of 1% to 4% N₂ in He by volume, astable discharge was possible at a pressure of 800 Torr or below.Greater concentrations of reactive gas (either N₂ or O₂) further loweredthe available operating pressures for stable discharges.

The gas used in the treatment of this invention is either helium alone,a mixture of helium and nitrogen, a mixture of helium and oxygen or amixture of helium, nitrogen and oxygen. If a mixture is used, it ispreferred to use helium with 0.1 to 8% nitrogen, helium with 0.1 to 8%oxygen or helium with 0.1 to 8% oxygen and 0.1 to 8% of nitrogen. Theseamounts are preferred as they give particularly strong adhesion atratios of speed to power density (measured in ft/min per W/cm²) of 1:1and greater and in the critical frequency range of 40 kHz to 500 kHz.

FIG. 2 shows a sectional view of the atmospheric glow dischargeapparatus used in the present invention. Electrode 20 includes a seriesof apertures 23 through which the gas is supplied through inlet 24. Thedimensions of electrode 20 are 12.1 cm by 13.3 cm. Electrode 20 has 333apertures 23 each having a 1 mm diameter. The apertures aresymmetrically distributed on the electrode 20. Surprisingly, it has beenfound that a stable glow discharge at atmospheric pressure with higherpercentages of reactive gas species, most notable N₂ and O₂, is possibleusing the electrode 20 shown in FIG. 2. This allows for a faster andmore complete treatment of the polyester substrate at low power. Theperforated electrode configuration shown in FIG. 2 can be operated inambient air with a mixture of 8% N₂ in He being supplied through theapertures 23. Using the solid electrodes of FIG. 1 a stable dischargewas not possible using the same gas mixture.

It is essential in the treatment of polymeric supports to enhance theadhesivity using a transport speed as high as possible with a power aslow as possible. As all photographic processes are carried out atextremely high speed to maintain adequate cost consideration and amplesupply, it is critical to be able to attain the desired adhesivity athigher web speed. This is extremely difficult as seen by the followingtable where mixtures of gas as described in U.S. Ser. No. 08/299,776were used at various speeds of from 1 to 30 ft. per minute. As the speedof the web increases many of the gas mixtures could not retain theadhesive properties.

It is also critical to use the lowest power possible because large powerrequirements increase the capital costs and can thermally damage the webbeing treated. The power density is defined as the power delivered tothe treatment electrode divided by the area of the treatment electrodeand is measured in watts per square centimeter.

It has been found herein that effective treatments at ratios of webspeed in ft./min. to power density in W/cm² of 1:1 or higher can beattained only at frequencies of from 40 kHz to 500 kHz.

In order to demonstrate the improved adhesion properties of PET and PENavailable from the method of the present invention comparative adhesiontests were run at different speeds, powers and frequencies using theperforated electrode of the present invention.

After treatment, the substrates (PEN and PET) were coated with a colorfilm emulsion. In addition to photographic emulsions other layers can beadhered to the substrate, such as antistatic, magnetic and lubricantlayers. Problems associated with electrostatic charge in the manufactureand utilization of imaging elements are well known. The accumulation ofcharge can result in dirt or dust attraction, producing physicaldetects. The discharge of accumulated charge during application or useof radiation sensitive layers (for example, photographic emulsions) canproduce irregular fog patterns or static marks in the light sensitivelayers(s). These static charge problems have become increasingly moresevere due to increased photographic emulsion sensitivity, increasedcoating machine speeds, and increased post-coating drying efficiency.Transport charging results from the tendency of high dielectricmaterials to accumulate electrical charge when in relative motion toother materials. This results in static charging during coating andpost-coating operations such as slitting and spooling. Static chargebuild-up may also occur during use of imaging elements system, forexample during winding of a roll of photographic film out of and backinto a film cassette in an automatic camera. Static discharge duringmagnetic reading and writing can result in increased bit error rates.These problems can be exacerbated at low relative humidities. Similarly,high speed processing of imaging elements can result in static chargegeneration. Due to the increasing demands for static charge control, awide variety of ionically-conducting and electronically-conductingmaterials have been incorporated into antistatic layers for photographicimaging, magnetic recording and other imaging elements.

As an example of auxiliary layers, that can be adhered to the polyestersubstrate, it is well known from various U.S. Pat. Nos., including3,782,947; 4,279,945; 4,990,276; 5,217,804; 5,147,768; 5,229,259;5,255,031; and others that a radiation-sensitive silver halidephotographic element may contain a transparent magnetic recording layerwhich can advantageously be employed to record information into and readinformation from the magnetic recording layer by techniques similar tothose employed in the conventional magnetic recording art. The use of amagnetic recording layer for information exchange allows improvedphotographic print quality through input and output of informationidentifying the light-sensitive material photographic conditions,printing conditions and other information.

Additional auxiliary layers may also be present in the imaging element.These layers may be used for but not limited to abrasion resistant andother protective layers, abrasive-containing layers, adhesion promotinglayers, curl control layers, transport control layers, lubricant layers,magnetic layers, and other layers for purposes such as improved webconveyance, optical properties, physical performance and durability.After the emulsion was set and dried a series of adhesion tape testswere run to test the adhesive properties of the treated PET and PEN.

An apparatus like that shown in FIG. 2 was operated at three frequencieswith several gases and gas mixtures. Polyethylene naphthalate wastransported through the treatment zone at several speeds to assess thecapability of the process to work in-line with other manufacturingoperations, such as the coating of photographic emulsions. The surfacesthus treated were then coated by hand with an anti-halation layer, whichis the first layer in many color photographic systems. In each case, theadhesion of the anti-halation layer was assessed in both the wet and drystates. Prior to testing, the coated films were dried for either 72hours (dry testing) or 336 hours (wet testing) at 22C. and 40% relativehumidity.

The dry test was done by attempting to peel the emulsion from thesupport with adhesive tape in five increasingly aggressive steps. Thesequence consists of changing the tape type, tape width, type of scoringtool, type of scoring, and tape peeling speed. Either a high speed steel(HSS) tool bit or a dissection scalpel is used to form the pattern inthe emulsion surface. A piece of the specific tape is then hand appliedand pressed onto the prepared area. The length of the leader, or pulltab, is test specific to further control the peel speed.

The tapes used include 810 (1/2 inch width), manufactured by 3M®company, 610 (1 inch width), and 396 (3/4 inch width). One of the toolbits may be used to slice the emulsion at the edge of the tape toconcentrate the peel stresses to the area under the tape. Or, the peelforces can be spread out by not scribing the edges. In each case, thetape is then peeled such that the peel angle is 90 degrees between thetape and substrate. The speed of the peeling motion is another factorwhich affects the aggressiveness of the particular test. Two of thetests utilize multiple peels to increase the aggressiveness. A summaryof the tests, in order of increasing aggressiveness is shown in Table

                  TABLE 1                                                         ______________________________________                                        Tape                              Edge        # of                            Test Tool     Pattern Tape  Leader                                                                              Slice Speed Peels                           ______________________________________                                        D    Scalpel  None    810   0.25" No    Slow  1                               E    Scalpel  None    810   0.25" Yes   Fast  1                               F    HHS Bit  H       810   4"    Yes   Fast  3                               G    Scalpel  #       610   4"    Yes   Fast  3                               H    Scalpel  #       396   2"    Yes   Fast  1                               ______________________________________                                    

The amount of the emulsion removed by the tape is recorded for eachcondition as a percentage of the original bounded area under the tape. Ascore of 0% removal means that no emulsion was removed under anycondition, and is considered necessary for product-quality photographicfilm. A score of 100% means that there was complete removal under all 5conditions. A score between 0 and 100% is determined by averaging theremoval for all 5 conditions.

The wet adhesion is assessed by placing the coated film in developersolution at a temperature of 38C. and rubbing it with an abrasive pad(Scotchbrite) while a pressure of 1.0N/cm² is applied to the pad. After60 back and forth cycles under the pad, the amount of emulsion removedis assessed as a percentage of the abraded area. A score of zero removalis considered necessary for product-quality photographic film.

Table 2 below summarizes the adhesion results for a variety of treatmentconditions, which use pure helium and mixtures of helium with nitrogen,oxygen, and carbon dioxide. For comparison, the results of coatingsdirectly on untreated support are shown. On support with no treatment,there is 100% removal in both the wet and dry tests, showing that theadhesion of photographic emulsions to untreated PEN is unacceptable.

                                      TABLE 2                                     __________________________________________________________________________    Gas    Power (W)                                                                           Speed (FPM)                                                                          ##STR1##    Frequency                                                                          Emul2, Dry                                                                          Emul2, Wet                         __________________________________________________________________________    1 He   700   1     .23          13.56M                                                                             32    100                                2 He   300   1     .54          13.56M                                                                             17    100                                3 2.0% N                                                                             300   1     .54          13.56M                                                                             3     81                                 4 He   600   1     .27          450K 0     0.7                                5 He   600   1     .27          450K 12    33                                 6 He   600   10    2.68         450K 64    100                                7 He   1,600 10    1.00         450K 0     0.1                                8 0.5% O                                                                             660   1     .24          450K 0     0                                  9 0.5% O                                                                             660   10    2.44         450K 0     3.4                                10                                                                              0.5% O                                                                             690   20    4.66         450K 0     100                                11                                                                              2.0% N                                                                             605   1     .26          450K 36    100                                12                                                                              2.0% N                                                                             605   10    2.66         450K 0     0                                  13                                                                              2.0% N                                                                             870   20    3.70         450K 0     0                                  14                                                                              2.0% N                                                                             1,950 30    2.47         450K 0     0.1                                15                                                                              1.2% CO.sub.2                                                                      500   1     .32          450K 0     0                                  16                                                                              He   700   1     .23           40K 13    36                                 17                                                                              3.0% O                                                                             300   1     .53           40K 0     66                                 18                                                                              3.6% N                                                                             300   1     .53           40K 0     0                                  19                                                                              3.6% N                                                                             300   5     2.68          40K 0     0                                  20                                                                              3.6% N                                                                             700   5     1.15          40K 0     0                                  21                                                                              AIR  300   1     .53          10K-CDT                                                                            0     3.6                                22                                                                              AIR  300   10    5.36         10K-CDT                                                                            4     100                                No Treatment                         100   100                                __________________________________________________________________________

Several important results are evident from the data in Table 2. First,the data reveal a surprising dependence of the adhesion results on thetreatment frequency. It is most easily seen by looking at runs 1,4, and16. All of these were done at the same speed, and comparable powers withpure helium gas. Run 1, done at a frequency of 13.5 mHz, has totallyunacceptable adhesion; run 4, done at 450 kHz has good adhesion; run 16,done at 40 kHz, has poor adhesion.

The criteria for viable products are that the speed/power density ratiomust be equal to or greater than 1 and the dry and wet adhesion removalscores must be less than 1 percent. Table 2 shows that the criteria areonly met within frequencies between 40 and 450 kHz. It is seen that byraising the power, excellent adhesion can be obtained using helium withnitrogen at 450 kHz operating at speeds up to 30 feet per minute. Thepoor performance of helium/nitrogen mixtures at 1 foot per minute underthese conditions could easily be due to too much treatment, which isknown to lead to a very damaged surface.

In order to demonstrate the results, the electrode used in theseexperiments was connected to a standard corona discharge treatment powersupply (10 kHz) and operated in ambient conditions, as is normally donewith CDT. It is seen from runs 21 and 22 that at 1 foot per minute theresults are completely unacceptable, for wet adhesion and at 10 feet perminute unacceptable for both wet and dry adhesion.

Roth et al (WO 94/28568) present an analysis of an atmospheric glowdischarge device in which they calculate a lower limit for the frequencyat which a discharge can be sustained. According to them, this frequencyis given by ##EQU1## where e is the ionic charge, V is theroot-mean-square discharge voltage, m is the ionic mass, δ is the ioniccollision frequency (given by Roth et al as 6.8×10⁹ per second) and d isthe plate separation for the discharge. At 40 kHz, the helium dischargesoperate at a plate separation of 1.5 mm with an rms voltage of 1100 V.According to Roth's teachings, the minimum frequency at which adischarge can be sustained under these conditions is 550 kHz. Some ofthe effective treatments herein, however, operate at 40 kHz, which isten times lower than the lower limit that Roth teaches.

These results demonstrate that treatments of polymer support in heliumor mixtures of helium with other reactive gases, done at the rightfrequencies, can significantly improve the adhesion of emulsion directlyto the support. These types of results are not possible withconventional corona treatments in air.

While the invention has been described with particular reference to apreferred embodiment, it will be understood by those skilled in the artthe various changes can be made and equivalents may be substituted forelements of the preferred embodiment without departing from the scope ofthe invention. In addition, many modifications may be made to adapt aparticular situation in material to a teaching of the invention withoutdeparting from the essential teachings of the present invention.

We claim:
 1. A method of treating a polymeric supportcomprising:providing a first electrode having a first surface, the firstelectrode having a plurality of spaced apart holes adjoining the firstsurface, the first surface being insulated; positioning a secondelectrode having a second surface spaced apart from the first surface ofthe first electrode; pumping a gas through the plurality of holeswherein the gas is greater than or equal to atmospheric pressure, thegas comprising helium; coupling a power supply to the first electrodehaving a frequency of between 40 and 500 kHz; connecting a power supplybetween the first surface of the first electrode and the second surfaceof the second electrode wherein the polymeric support is subjected toatmospheric glow discharge and maintaining a ratio of web transportspeed in ft./minute of the polymeric support to the power density in thetreatment zone in watts per square centimeter of at least 1:1.
 2. Themethod of claim 1 wherein the gas comprises helium and oxygen.
 3. Themethod of claim 1 wherein the gas comprises helium and nitrogen.
 4. Themethod of claim 1 wherein the gas comprises helium, oxygen and nitrogen.5. The method according to claim 3 wherein the nitrogen content isbetween 1% and 4% by flow.
 6. The method of claim 5 wherein the nitrogencontent is 2% by flow.
 7. The method of claim 2 wherein the oxygencontent is between 0.1% and 8% by flow.
 8. The method of claim 1 whereinthe first electrode comprises aluminum.
 9. The method of claim 8 whereinthe first surface is insulated by modifying the aluminum.
 10. The methodof claim 1 wherein the polymeric support comprises a polyester.
 11. Themethod of claim 10 wherein the polyester is polyethylene terephthalate.12. The method of claim 10 wherein the polyester is polyethylenenaphthalate.
 13. The method of claim 1 wherein the polymeric support ispolyethylene coated paper.
 14. The method according to claim 1 furthercomprising;coating the polymeric support with a photographic emulsionantistatic layer, magnetic layer or lubricant layer after the polymericsupport is subjected to the atmospheric glow discharge.
 15. The methodof claim 14 wherein the polymeric support is first treated with a layerof gelatin and then coated with the photographic emulsion.
 16. Themethod of claim 14 wherein the support is coated with an antistaticlayer.
 17. The method of claim 14 wherein the support is coated with amagnetic layer.
 18. The method of claim 14 wherein the support is coatedwith a lubricant layer.